GLACIER

Cavity-Ringdown (CRD) Loss Meter and Reflectometer

Our reflectometer GLACIER uses the extreme sensitivity of cavity ring-down spectroscopy to 

quantify the losses of advanced optical coatings down to 5 ppm. As a typical application the 

device can characterize supra-mirrors with up to 99.9995 % reflectivity. Conventional 

absorption and reflection measurements are not sufficiently sensitive to quantify today´s 

super-reflective mirror coatings and are typically limited to the > 1000 ppm range 

(corresponding to < 99.9 % reflectivity). Cavity ring-down spectroscopy measures optical

losses by the decay of the energy stored inside a cavity. The technique reaches unrivalled 

sensitivity, because losses are experienced with each round trip inside the cavity over and over 

again. Smaller losses lead to longer intra-cavity dwell time thereby automatically increasing 

measurement precision. The device features high-speed data acquisition and allows to record 

measurements within seconds. It is delivered complete with a computer and a user-friendly 

software interface for acquisition and real-time analysis.

Sketch of the working principle of GLACIER, measurement and fitting procedure

Sample Measurement

The device measures the time-dependent intensity I(t) leaked through an end mirror 

of the cavity (center). 

The signal decays with a time constant depending on the intra-cavity losses and can

be fitted to the following exponential function:

The time constant τ is inversely proportional to the optical losses (1-R) of the cavity 

withtotal reflectivity R:

where n is the refractive index, c is the speed of light, and l is the cavity length

Typical GLACIER measurement of an ultrahigh-reflective mirror for 1030 nm. 

To obtain the data, the cavity losses with and without the sample were 

measured and subtracted. This provides an absolute measurement 

of the test mirror